Uninterruptible power supply (UPS) systems are commonly used for computers, fax machines, and other electronic devices. These systems provide protection against primary AC power failure and also against variations in power line frequency and voltage. There are generally three types of UPS systems: off-line or standby; line interactive; and on-line.
Off-line UPS systems do not regulate output voltage when the load is operating on utility power. As a result, off-line UPS systems are ineffective during power surges, spikes and brownouts, i.e. periods when a voltage reduction is initiated by a utility to counter excessive demand on its electric power generation and distribution system. In addition, when AC utility power is lost, an off-line UPS system requires a short transfer time before switching to battery power. These transfer times are typically several milliseconds, which makes off-line UPS devices unsuitable for use with sensitive equipment.
Line interactive UPS systems regulate voltage by adjusting the utility voltage before it passes to the load, and thus provide protection during brownouts and against power surges and spikes. However, these systems still exhibit transfer times of the order of a millisecond, and therefore are also unsuitable for use with sensitive equipment.
On-line UPS systems are connected between the power line and the load to provide for continuous voltage regulation and suppression of transients and noise. The transfer times for such systems are extremely small, and so they are suitable for very sensitive or highly critical equipment. In addition, on-line UPS systems provide for an improved and more efficient utilization of input utility power, and the improved power factor helps lower energy costs.
UPS systems, especially on-line systems, are particularly valuable with respect to computer systems since they provide users with data and equipment protection. A significant number of computer system breakdowns are caused by utility power failures and fluctuations, and this may require that expensive hardware be replaced and software reinstalled. Power problems also often lead to lockups, crashes, lost data, and faulty data transmissions. For instance, users need power protection to ensure that an electronic data transfer via the internet is properly completed, without any loss of data or sensitive information, even in the event of a power failure. A significant amount of time may be expended and business lost in attempting to recover from power disruptions. Computers also now perform multiple functions such as sending faxes and answering telephone calls, making the need for power protection even greater.
Also, although power grids used by businesses may be more reliable than power grids for residential purposes, more and more people have begun working out of their homes. In addition, utility power supply in developing nations can be very inconsistent, with disruptions often occurring several times a day.
Typically, a UPS system includes a storage battery, a battery charger, a switching circuit, and an inverter circuit which converts a DC voltage into an AC voltage. Conventional inverter circuits typically employ push-pull circuitry to alternately drive the two primary windings of a transformer and generate an alternating positive/negative or AC signal at the secondary windings of the transformer. This type of inverter is widely used in small and medium UPS systems (i.e. below 1 kVA (kilo-voltamperes)), as well as in other equipment which converts DC voltage to AC voltage. Although such inverters are simple and reliable, they typically require low frequency (10-100 Hz) iron core transformers which are large, heavy, and expensive.
As a result, UPS systems which incorporate these inverters are large, bulky, and generally unsuitable for use with personal computers, fax machines, and other equipment requiring small UPS systems. Thus, while computer network server computer systems are often protected by UPS technology, this is much more rarely the case for individually based computers, despite the significant benefits provided by UPS systems. Current UPS technology is also expensive, particularly for on-line UPS systems.
There is therefore a need for a novel inverter circuit which operates in a reliable and stable manner but does not require an iron core transformer, and which can consequently be incorporated into a UPS device which is significantly smaller in size, lighter in weight, and more cost efficient than conventional UPS devices. Such a UPS system would provide further practical benefits if it could be integrated with a conventional power supply for specific types of electronic equipment, such as a personal computer.